During recent years, many efforts to increase the capacity of optical networks aim at improving the spectral and temporal efficiency of optical communications signals. One method consists in using Sinc pulses, which have the unique property of exhibiting a rectangular spectrum in the frequency domain and satisfy the Nyquist inter-symbol interference (ISI) criterion in the time domain. This means that the data symbols that are carried by Sinc pulses do not interfere with each other, even though the pulses overlap in the time domain. These properties allow for an optimized aggregation in the frequency domain of wavelength division multiplexed (WDM) channels, as well as an orthogonal multiplexing of the symbols in the time domain with no ISI.
The most common technique to generate Nyquist signals in optical telecommunications networks is Nyquist WDM, where the baseband communication signal spectrum to be transmitted is reshaped using digital Nyquist filters before modulation on an optical carrier. Transmission of WDM channels with no guard band and no ISI was demonstrated using Nyquist WDM. The drawback of this method however is the high signal processing capacity needed in order to generate Nyquist WDM signals at high speed. Thus, up to now, Nyquist WDM systems with rather low bandwidths have been demonstrated.
Another approach is to generate Nyquist pulse-sequences in the optical domain. In this approach, an optical Nyquist pulse-sequence is generated and used as the symbol to transmit data on an optical link. The pulse can then be modulated with data and multiplexed in time and/or frequency domain with no ISI and no guard band. Apart from reducing the need for high speed signal processors, the advantage is that one can directly use the generated pulses for other applications but telecommunications, such as optical sampling or sensing. Currently existing methods to generate all-optical Nyquist pulses use expensive equipment, such as mode-locked lasers or optical parametric amplifiers. Moreover, the all-optically generated Nyquist pulses demonstrated so far are not close to the ideal Sinc pulses: in particular they do not exhibit a high quality rectangular spectrum.